It is known for titanium nitride coatings to be applied to injection molding tools for the injection molding of plastic materials, in order to reduce the level of coating deposit thereon, to improve the ease with which moldings can be removed from the molding tools, and to reduce the amount of tool wear. It has been found however that those titanium nitride coatings are often not satisfactory in regard to their properties.
Coatings which are found to be an improvement in comparison with such titanium nitride coatings, in terms of the surface properties thereof, are those which contain chromium nitride as an essential constituent thereof. Chromium nitride-bearing coatings however suffer from the disadvantage that the most important properties thereof tend to partially vary during the service life of the molding tools, for example when assembling or vulcanizing rubber parts in which the chromium nitride coatings lose their adhesion-inhibiting effect after a short period of operation.
It is also known that zirconium nitride coatings can be applied to articles in some cases as decorative coatings, by virtue of their brass colorations. Those coatings admittedly in themselves form a hard surface and are chemically resistant to lyes, but they have in particular metal bonds.
Reference may be made to EP-A-0 603 486 describing a multi-stage process for coating cutting and shaping tools with ternary hard-material layer assemblies or laminate structures such as titanium aluminum nitride, zirconium aluminum nitride or chromium aluminum nitride. Although those layer materials are admittedly suitable for tools for metal machining, they cannot however be used to real effect for reducing contact adhesion and for enhancing the chemical resistance of sensitive surfaces such as for example plastic material extrusion tools which are polished to a high state of shine.
With those materials the solid laminate components involve predominantly metal bonds, in which respect thorough and extensive scientific tests have demonstrated that all materials involving a predominantly metal bonding effect have a tendency to suffer from contact adhesion to a greater or lesser degree.